Iswan, If you tell us where you live, local quilters in your area might be able to help with specific sources for cotton plaid shirts.
I have found that yard sales are not worth my trouble as the individuals selling want to get a good price for the shirts they purchased and are now selling. . . Church rummage sales are a very good source, however, as the for-sale items have been donated, and any money raised is totally profit.
I live in Dallas, and find lots of cotton plaid shirts at the Thrift Giant. I buy them during 50% off days and pay $2 - $3.50 each. Yes, I read about quilters who purchase shirts in their localities for a lot less and I swoon. . .